The sun emits a wide optical spectrum that peaks in the visible and has 60% of its photon flux in the wavelength range spanning from ˜350 nm to ˜1350 nm. This wavelength range corresponds to 80% of the sun's total power flux of ˜1.3 kW/M2 at the earth.
It has been known for decades that the best approach to convert the sun's optical power into electrical power is through solar cells that make use of absorption transitions in semiconductors. Photon energy is harnessed in this way by exciting electrons from the semiconductor's valence band across the bandgap into the conduction band. The photocarriers thus generated, i.e. the electrons and holes, are then swept across a p-n or p-i-n junction fabricated by doping different regions of the semiconductor structure, and are used to produce electricity. Semiconductors or semiconductor alloys with bandgaps Eg absorb impinging photons having energies greater than or equal to Eg as opposed to photons having energies less than Eg. Equivalently, it can be said that photons having wavelengths corresponding to energies greater than Eg are absorbed while photons having longer wavelengths are not.
Since the energy of a photon in excess of Eg is effectively lost through thermal processes, it is well established that a combination of materials having different bandgaps must be used to adjust the voltage and current of the solar cell in order to optimize the conversion efficiency of solar light into electricity. To that effect, multijunction solar cells, also known as tandem solar cells, have been developed for applications requiring higher conversion efficiencies.
From a fabrication and crystal perspective, the choice of semiconductors or semiconductor alloys is practically restricted to materials that can be grown on common substrates, such as GaAs, Ge, Si, or InP substrates, with a minimum of defects. To date, the best optical to electrical conversion efficiencies, the conversion efficiency being defined as the electrical power that the device can produce divided by the optical power received from a light source such as, for example, the sun, are around 30% and have been achieved by growing a monolithic multijunction cell having a GalnP top subcell (Eg˜1.8 eV), a GaAs middle subcell (Eg˜1.4 eV), and a Ge bottom subcell (Eg˜0.7 eV) on a Ge substrate. Since the subcells are typically connected in series through tunnel junctions, it is recognized that to improve further the conversion efficiency, the bandgaps of the materials have to be changed or, a fourth subcell added.
The total voltage of the multijunction cell is essentially the sum of the voltages generated by the individual subcells, where the voltage of each subcell is proportional to the subcell's bandgap. To optimize the conversion efficiency, the subcells should be current-matched, otherwise the subcell generating the weakest current limits the overall current. In the case above, GalnP has a bandgap that can absorb 25% of the total solar photon flux (sometimes referred to as the AM0 spectrum), whereas only 14% of the total solar photon flux transmitted through the GalnP subcell can be absorbed by the GaAs subcell, and 38% of the total solar photon flux transmitted through the GaAs subcell can be absorbed by the Ge subcell.
This clearly leads to a current imbalance in the multijunction cell. Relatively speaking, the GaAs subcell does not absorb enough solar photons while the Ge subcell captures too many. To equilibrate the current balance between the subcells, the middle subcell, i.e. the subcell disposed between the GalnP and the Ge subcells, would have a smaller bandgap. For example, a middle subcell having a bandgap of ˜1.16 eV (corresponding to an optical wavelength of approximately 1100 nm) would imply that all the three subcells would each absorb approximately 25% of the total solar photon flux. The remaining 25% of the solar photon flux would not be absorbed since the three subcells are transparent to the longer wavelength photons (i.e. photons with wavelengths greater than 1.8 μm are not absorbed).
As mentioned above a four-subcell arrangement can improve the current balance. If a material with Eg˜1.0 eV is introduced between the GaAs and the Ge subcells, it yields the following distribution in the absorption of the solar photon flux: ˜25% of the photons absorbed by the first subcell (GalnP), ˜14% by second subcell (GaAs), ˜19% by the third subcell (Eg ˜1 eV), and ˜19% by the fourth subcell (Ge). However, this four-subcell arrangement is still current-limited by the GaAs subcell. To make the four-subcell arrangement better balanced in terms of current, the thickness of the first subcell can be adjusted (reduced) to let some of the shorter wavelength photons reach the second subcell. In this scenario, the second subcell absorbs more photons having energies greater than that of its bandgap thus leading to more thermally wasted energy. This was described by Olson et al. in U.S. Pat. No. 5,223,043 incorporated herein by reference.
Research and development to find new materials and novel multijunction arrangements to improve the efficiency of solar cells has been very active. For example, Olson, in U.S. Pat. No. 4,667,059, disclosed dual GalnP/GaAs cells on a GaAs substrate; Ho et al., in U.S. Pat. No. 5,405,453, disclosed dual GalnP/GaAs cells on a Ge substrate; Wanlass, in U.S. Pat. No. 5,019,177, disclosed dual InP/GalnAsP cells on InP; Freundlich et al., in U.S. Pat. No. 5,407,491, disclosed dual InP/InGaAs cells on an InP substrate; Chang et al., in U.S. Pat. No. 5,330,585, disclosed the dual AlGaAs/GaAs cells on a GaAs substrate; these patents being incorporated herein by reference.
These examples of dual cells and the triple cell made of GalnP/GaAs/Ge on a Ge substrate can have conversion efficiencies close to 30% as long as compromises in the design or in the quality of the materials are made. The compromise in the case of the dual cells having GaAs as the smallest bandgap is that the longer wavelength photons are not absorbed, they are transmitted though all the layers. In the case of dual cells with the smaller InGaAs or InGaAsP bandgaps, the compromise is that the shorter wavelength photons are losing their excess energies in heat. It is also worth nothing that GaAs or Ge substrates have the advantage of a lower cost compared to InP substrates.
To extend the photo-absorption of GalnP/GaAs cells to longer wavelengths, Freundlich, in U.S. Pat. No. 6,372,980, incorporated herein by reference, disclosed solar cells with InGaAs quantum wells, the solar cells having modeled efficiencies in excess of 30%. Other schemes have also been disclosed to try to improve the efficiency of solar cells. For example, Freundlich et al., U.S. Pat. No. 5,851,310, incorporated herein by reference, disclosed the use of strained quantum wells grown on an InP substrate. Also, Suzuki in U.S. Pat. No. 6,566,595 (later referred to as '595), incorporated herein by reference, disclosed the use of quantum well layers having a plurality of projections with different sizes, the goal being of better matching the sun's spectrum by using materials having different bandgaps.
Similar is the disclosure by Sabnis et al. in U.S. Pat. No. 4,206,002, incorporated herein by reference, for bulk graded bandgap multijunction solar cells. In the case of the '595 patent however, the overall efficiency is unlikely to be improved since the tailoring of the absorption spectrum involves distributing quantum well or quantum dot materials of different sizes in the plane of the layers. This compromises the spatial density of the material that can be used to absorb light and is likely to require thicker layers to absorb the same number of photons as would be absorbed in uniform layers or, larger surfaces which would reduce the conversion efficiency. Chaffin et al., in U.S. Pat. No. 4,688,068, incorporated herein by reference, also disclosed the use of quantum wells in multijunction cells.
As disclosed by Kurtz et al. in U.S. Pat. No. 6,252,287, incorporated herein by reference, InGaAsN lattice-matched to GaAs is also a promising material for tailoring the bandgap of layers lattice-matched to GaAs for optimizing the conversion efficiency.
Other aspects of the fabrication of monolithic multijunction solar cells such as antireflection windows, tunnel junctions, and surface metallization have matured with the extensive developments of photovoltaic solar cells as disclosed in numerous patents and publication in that field as seen in several of the patents identified above (for example, U.S. Pat. Nos. 4,694,115; 5,009,719; 4,419,530; 4,575,577).
In the field of semiconductor nanostructures, it is well known that high-quality, defect-free, self-assembled quantum dots can be obtained during the early stage of growth of highly strained semiconductors (see for example: S. Fafard, et al., “Manipulating the Energy Levels of Semiconductor Quantum Dots”, Phys. Rev. B 59, 15368 (1999) and S. Fafard, et al., “Lasing in Quantum Dot Ensembles with Sharp Adjustable Electronic Shells”. Appl. Phys. Lett. 75, 986 (1999)). Such quantum dot material can be grown in multiple layers to achieve thick active regions for devices such as Quantum Dot Infrared Photodetectors, as disclosed by Fafard et al. in U.S. Pat. No. 6,239,449, incorporated herein by reference. There, the interband absorption properties of the quantum dot material can be tailored to cover various wavelength ranges in the near infrared and visible portions of the optical spectrum. The composition, size and shape of the quantum dot material are adapted to change the quantization energies and the effective bandgap of the quantum dot material, where the effective bandgap of the material is defined as essentially being the lowest energy transitions at which photons can be absorbed and is determined by the quantized energy levels of the heterostructure.
Self-assembled quantum dots come in a wide range of high quality materials that can be pseudomorphically grown on GaAs or InP. For example, InAlAs/AlGaAs on GaAs substrates absorbs in the red or the near-infrared, InAs/InAlAs on InP substrates absorbs in the 1.5 μm wavelength range, and InAs/InGaAs on InP substrates absorbs in the 1.9 μm wavelength range. More importantly however, In(Ga)As/GaAs self-assembled quantum dot material grown on GaAs substrates, is particularly well-suited for absorption below the GaAs bandgap in the spectral region spanning from 885 nm to 1150 nm, or up to ˜1350 nm depending on the growth parameters. In(Ga)As/GaAs self-assembled quantum dot layers can be grown uniformly and with high densities. Furthermore, multiple layers can be grown with the same uniformity or, when desirable, with different sizes and/or compositions by simply controlling the growth parameters. Additionally, the In(Ga)As/GaAs self-assembled quantum dot material has been shown to produce devices which are orders of magnitude more radiation robust than conventional material (see for example: P. G. Piva et al., “Enhanced Degradation Resistance of Quantum Dot Lasers to Radiation Damage”. Appl. Phys. Left. 77, 624 (2000)). The radiation and defect hardnesses are particularly great advantages for space applications where the solar cells are getting exposed to radiations.
As can be appreciated from the prior art discussed above, there is a real need for high quality materials having desired absorption spectra, that can be easily incorporated in multijunction solar cells to improve further the conversion efficiency. A reliable material that can balance the absorption between the bandgaps of GaAs and Ge is of particular interest.